1. Field of the Invention
The present invention relates to a device for adjusting the null value of a linear variable differential transformer (LVDT)
2. General Background and State of the Art
A linear variable differential transformer (LVDT) is a displacement transducer that produces an electrical signal proportional to the displacement of a moveable core (armature) within a cylindrical transformer. The transformer consists of a central, primary coil winding and two secondary coil windings on opposite ends of the primary winding. The coil windings are coaxial. The armature preferably is nickel-iron and is positioned within the coil assembly. The core provides a path for magnetic flux linking the primary coil to the secondary coils.
When the primary coil is energized with an alternating current, a cylindrical flux field is produced over the length of the armature. This flux field produces a voltage in each of the two secondary coils that varies as a function of the armature position. Armature movement moves the flux field into one secondary and out of the other causing an increase in the voltage induced in one secondary and a corresponding voltage decrease in the other. The secondary coils are normally connected in series with opposing phase. The net output of the LVDT is the difference between the two secondary voltages. When the armature is positioned symmetrically relative to the two secondaries (the xe2x80x9cnullxe2x80x9d position), the differential output is approximately zero, because the voltage of each secondary is equal but of opposite phase.
Subjecting a transducer to pressure can move an LVDT armature through a linkage. As pressure increases, the armature moves toward one secondary winding and away from the other. This yields a voltage difference that can be proportional to the pressure on the transducer. Consequently, this voltage output can measure pressure and position.
Nearly all LVDTs that are designed for aircraft or missile applications are wound on an insulated stainless steel spool, magnetically shielded and enclosed in a stainless steel housing using welded construction. The armature is normally made from a 50% nickel-iron alloy and brazed to a stainless steel extension. Secondary leads are usually shielded to minimize channel-to-channel crosstalk for multi-channel units and to shield components from RF energy.
The length and diameter of an LVDT must be sufficient to allow adequate winding space for achieving the desired electrical performance, support any pressure requirement and withstand the environmental shock, vibration and acceleration. Where physical size is limited, electrical performance must be flexible. Although the LVDT is basically a simple device, the operating characteristics and electrical parameters are complex and depend to a large extent on the physical limitations.
The minimum diameter of the transformer housing will depend on electrical performance criteria for the excitation frequency being used and housing wall thickness required to support a pressure requirement. Armature diameters less than 0.110 in. (2.8 mm) (metric conversions are approximate) are easily damaged and are not recommended. The armature and the probe to which the armature is attached mounts within a tubular member. The probe and tube through which the armature moves should be slightly larger in diameter than the armature to protect the armature from rubbing against the tube.
The full scale or span is the displacement range of the LVDTs armature for which the electrical performance is required and is referred to as the electrical stroke. Since an LVDT is normally, although not necessarily, a center null device (zero output occurs at mid-stroke), the range or stroke is normally specified as a plus and minus displacement from the null position. The full stroke (100% of the stroke) is the total end-to-end stroke, and the full scale output (100% of the output) is the total end-to-end output voltage.
An LVDT requires an AC voltage for operation. This excitation could be provided by aircraft buss power or an excitation source specifically designed for an LVDT. In today""s aerospace and aircraft industry, multi-channels with individual excitation sources are often used to obtain the highest possible system reliability.
An LVDT""s output voltage is pro portional to the voltage applied to the primary. System accuracy depends on providing a constant input to the primary or compensating for variations of the input by using ratio techniques. The output can be taken as the differential voltage or, with a center tap, as two separate secondary voltages whose difference is a function of the displacement. If the sum of the secondary voltages is designed to be a specific ratio of the difference voltage, overall accuracy significantly improves.
Resolution of an LVDT is the smallest change in armature position which can be detected as a change in the output voltage. Sub-micro-inch resolution is not uncommon with LVDTs. In practice, the resolution is usually less than the noise threshold of external circuitry or resolution of the equipment used to measure the output.
Where system reliability requires more than one output signal for redundancy, up to four independent LVDTs can be packaged in a single transducer assembly. Coil placement may be in series or grouped side-by -side as a cluster. Multiple LVDT""s in one housing require less space, weight, installation time and cost less than separately mounted LVDTs. Dual LVDT assemblies are two coils combined, in tandem or parallel. The choice of the configuration could be limited by the length or diameter of the envelope available for the installation. Triple LVDT assemblies are usually combined in parallel. The tandem configuration is excessively long for strokes above one inch. Finally, quad LVDT assemblies are nearly always combined in parallel.
Having an accurate starting point for the LVDT is necessary for accurate measurement. That starting point has the armature centered and the differential output is zero. When that occurs, the armature is in its null position.
The prior art recognizes the advantages of being able to adjust that null position. Examples include. Kather, U.S. Pat. No. 5,804,962 (1998), and Maples, U.S. Pat. No. 4,543,732 (1985).
In some cases it is desired to use an LVDT to determine the position of a linkage in a pressurized zone, and to be able to adjust the null position of the LVDT. Adjusting the null position may be accomplished by having the magnetic element of the LVDT move within a sealed tube, with the tube extending outward from the pressurized zone. Adjusting the null point of the LVDT without removing the sealed tube is desirable. It also is important not to place torsional stress on the tube that holds the pressure while the null is being adjusted.
One object, therefore, of the present invention is to provide an LVDT with simplified null adjustments. One problem with adjusting the null position relates to the assembler. It is an object of the present invention to enable an assembler to assemble a sensor, pressurize the system and then adjust the null position. One problem with some LVDT is that adjustments to the null position apply torque to one or more of the parts. That torque can lead to inaccurate measurements and could rupture the seal of the sealed tube in which the magnetic element moves. Therefore, it is an object of the present invention to provide a null adjustment mechanism that does not apply torque to the sealed tube. Another object of the present invention is to disclose and provide a method for adjusting the null position of an LVDT such that the coil assembly is moved relative to the LVDT assembly.
Many systems for adjusting the null position do so by positioning the armature. It is an object of the present invention to avoid moving the armature during the adjustment procedure.
In the present invention, the LVDT coils themselves are mounted for movement relative to an armature that remains fixed during the null adjusting step. The tube in which the armature mounts and the armature itself are fixed, and the support for the coils can be adjusted axially relative to the armature and its supporting tube. This adjustment is preferably through a threaded connection at the outer end of the coil assembly. Springs bias the coil support to maintain tightness in the system and to avoid backlash.
In one illustrative embodiment of the invention, the linear variable differential transformer (LVDT) assembly with nulling adjustment comprises a housing defining a pressure barrier between first and second zones. An armature tube is sealed to the housing at its inner end. The tube has an outer end that extends into the second zone and a central section. An LVDT multiple coil assembly extends around the central section of the tube and is movable longitudinally relative to the tube. A magnetic armature mounts on a mechanical linkage and extends into the tube within the multiple coil assembly. A fixed support in the housing extends around the tube near the tube""s outer end. The coil assembly is adjustable axially along the tube and armature. A spring biases the coil assembly away from the fixed support toward the inner end of the tube. In one embodiment, the adjusting mechanism includes a threaded member secured to the coil assembly and extending along the outer end of the tube through the fixed support. An adjusting nut on the other side of the fixed support from the housing engages the threaded member and bears on the fixed support as a result of the biasing force of the spring.